The present invention relates generally to steam turbines, and more particularly to a method for increasing the power output of an existing steam turbine. A steam turbine has a steam path which typically includes, in serial-flow relationship, a steam inlet, a turbine, and a steam outlet. A gas turbine has a gas path which typically includes, in serial-flow relationship, an air intake (or inlet), a compressor, a combustor, a turbine, and a gas outlet (or exhaust nozzle). Gas or steam leakage, either out of the gas or steam path or into the gas or steam path, from an area of higher pressure to an area of lower pressure, is generally undesirable. For example, gas-path leakage in the turbine or compressor area of a gas turbine, between the rotating rotor shaft of the turbine or compressor and the circumferentially surrounding turbine or compressor casing, will lower the efficiency of the gas turbine leading to increased fuel costs. Also, steam-path leakage in the turbine area of a steam turbine, between the rotating shaft of the turbine and the circumferentially surrounding casing, will lower the efficiency of the steam turbine leading to increased fuel costs.
The turbine portion of a steam turbine typically includes a high-pressure turbine section, perhaps an intermediate-pressure turbine section, and a low-pressure turbine section each having rotatable steam-turbine blades fixedly attached to, and radially extending from, a steam-turbine shaft which is rotatably supported by bearings. The bearings usually are located longitudinally outward from the high and low-pressure turbine sections and longitudinally between the high and intermediate-pressure turbine sections. Usually the steam pressure drop through a high-pressure or an intermediate-pressure turbine section is a least generally 2,000 kPa (kiloPascals), and the difference in pressure of the steam entering the high and intermediate-pressure turbine sections is at least generally 600 kPa. It is noted that the steam exiting the high-pressure turbine section normally is reheated by the boiler before entering the intermediate-pressure turbine section and that the "steam" exiting the low-pressure turbine section enters a condenser before being directed to the boiler, as is known to the artisan.
Labyrinth seals have been used to minimize steam-path leakage between the high-pressure turbine section and its longitudinally-outward bearing, between the low-pressure turbine section and its longitudinally-outward bearing, and between the high and intermediate-pressure turbine sections. Such labyrinth seals consist of longitudinally spaced-apart rows of labyrinth seal teeth. Many rows of teeth are needed to seal against the high-pressure differentials found in a typical steam turbine, and this has contributed to the impressive longitudinal length (sometimes over fifteen meters) of a standard steam turbine used by power utilities to turn a generator for the production of electricity. Such length has required extra bearings to support the steam-turbine rotor.
It is noted that brush seals are used to minimize leakage through a gap between two components, wherein such leakage is from a higher pressure area to a lower pressure area. Brush seals have been used, or their use proposed, in rotating machinery. Such use or proposed use includes, but is not limited to, turbomachinery including steam turbines and gas turbines used for power generation and gas turbines used for aircraft and marine propulsion. It is noted that brush seals minimize the leakage of steam in steam turbines and minimize the leakage of compressed air or combustion gases in gas turbines.
The brush seals usually have wire or ceramic bristles conventionally welded or otherwise affixed to a backing plate. To improve performance, such brush seals typically align their wire bristles to contact the rotating rotor shaft at an angle between generally forty-five and generally sixty degrees with respect to a radius line from the center of the rotor to the point of bristle contact. For high pressure applications, a seal plate is positioned against the seal bristles on the downstream side (i.e., lower-pressure side) of the brush seal. Brush seals do a better sealing job than labyrinth seals, as can be appreciated by the artisan. However, all known steam turbines or steam turbine designs which rely on a brush seal assembly between turbine sections or between a turbine section and a bearing also rely on a standard labyrinth seal which acts as a backup seal for the brush seal assembly.
In overhauling a steam turbine, it is noted that an installed steam turbine has an existing length which is a fixed maximum length which cannot be made longer because of a connected generator and nearby associated equipment, building walls, etc. What is needed is a method for overhauling a steam turbine to increase its power without increasing its overall length.